It is known that the mechanical properties of gelatin containing layers of photographic materials can be improved by addition of a hardening agent. In fact, photographic materials are usually stored at elevated temperatures and humidity conditions or treated with various aqueous solutions having different pH and temperatures, and gelatin layers which have not been treated with a hardening agent have poor water resistance, heat resistance and abrasion resistance.
Many compounds are known to be effective for increasing mechanical resistance of a gelatin layer by hardening. They include, for example metal salts such as chromium, aluminium and zirconium salts; aldehydes such as formaldehyde and glutaraldehyde; halogenocarboxylaldehydes such as mucochloric acid; aziridine compounds such as those described in U.S. Pat. No. 3,017,280; epoxy compounds such as those described in U.S. Pat. No. 3,091,537; halogenotriazine compounds such as hydroxydichlorotriazine and aminodichlorotriazine; and compounds having vinylsulfonyl groups such as methylene-bis-vinylsulfone, divinylsulfone and methylene-bis-vinylsulfonamide.
A group of hardening agents for photographic gelatin-containing layers which is particularly interesting has been disclosed in U.S. Pat. No. 4,063,952. These hardening agents are carbamoyl pyridinium compounds in which the pyridine ring carries a sulfo or sulfoalkyl group. These compounds have high water solubility, a fast hardening action for gelatin, and low occurrence of post-hardening (post-hardening is a change in the degree of hardening caused by slow continued hardening of the gelatin). They belong to the group of "quick-acting" hardeners with which light-sensitive photographic materials can be hardened to an optimum degree within a very short time.
JP 05/265,115 describes a silver halide photographic material comprising tabular grain emulsions, a carbamoyl pyridinium hardener and at least one erythritol compound to improve the covering power and sensitivity in rapid processing.
EP 578,191 discloses the use of pyridinium hardener in combination with copolymers based on acrylamide and sulfoalkylacrylamide monomers in silver halide photographic materials to improve the coatability of the silver halide emulsion layer(s).
JP 05/119,427 describes a silver halide photographic material comprising at least one trisubstituted triazine and at least one pyridinium sulfonate in emulsion and/or hydrophilic layer to improve the storage stability at high temperatures of the photographic material.
However, when the carbamoyl pyridinium hardeners are used with conventionally known silver halide photographic elements, i.e, photographic elements comprising isometric (cubic, octahedral, end the like) silver halide grains and/or conventional gelatin, they give unsatisfactory results. In particular, the physical properties of roller marking and hard mottle were worse than those obtained with conventional hardener, and there was no additional benefit in terms of sensitometric results.